The present invention relates generally to plasma excitation coils and, more particularly, to an excitation coil having at least one capacitance connected across a discontinuity between the coil excitation terminals, and to a workpiece processor including such a coil. The invention also relates to a method of operating an excitation coil such that a standing wave has a sudden amplitude and slope change, as well as a sudden slope reversal, between the coil excitation terminals.
One type of processor for treating workpieces with an RF plasma in a vacuum chamber includes a coil responsive to an RF source. The coil responds to the RF source to produce electromagnetic fields that excite ionizable gas in the chamber to a plasma. Usually the coil is on or adjacent to a dielectric window that extends in a direction generally parallel to a planar horizontally extending surface of the processed workpiece. The excited plasma interacts with the workpiece in the chamber to etch the workpiece or to deposit material on it. The workpiece is typically a semiconductor wafer having a planar circular surface or a solid dielectric plate, e.g., a rectangular glass substrate used in flat panel displays, or a metal plate.
Ogle, U.S. Pat. No. 4,948,458 discloses a multi-turn spiral coil for achieving the above results. The spiral, which is generally of the Archimedes type, extends radially and circumferentially between its interior and exterior terminals connected to the RF source via an impedance matching network. Coils of this general type produce oscillating RF fields having magnetic and capacitive field components that propagate through the dielectric window to heat electrons in the gas in a portion of the plasma in the chamber close to the window. The oscillating RF fields induce in the plasma currents that heat electrons in the plasma. The spatial distribution of the magnetic field in the plasma portion close to the window is a function of the sum of individual magnetic field components produced by each turn of the coil. The magnetic and electric field components produced at each point along the coil are respectively functions of the magnitude of RF current and voltage at each point. The current and voltage differ for different points because of transmission line effects of the coil at the frequency of the RF source.
For spiral designs as disclosed by and based on the Ogle ""458 patent, the RF currents in the spiral coil are distributed to produce a torroidal shaped magnetic field region in the portion of the plasma close to the window, which is where power is absorbed by the gas to excite the gas to a plasma. At low pressures, in the 1.0 to 10 mTorr range, diffusion of the plasma from the ring shaped region produces plasma density peaks just above the workpiece in central and peripheral portions of the chamber, so the peak densities of the ions and electrons which process the workpiece are in proximity to the workpiece center line and workpiece periphery. At intermediate pressure ranges, in the 10 to 100 mTorr range, gas phase collisions of electrons, ions, and neutrons in the plasma prevent substantial diffusion of the plasma charged particles outside of the torroidal region. As a result, there is a relatively high plasma flux in a ring like region of the workpiece but low plasma fluxes in the center and peripheral workpiece portions.
These differing operating conditions result in substantially large plasma flux (i.e., plasma density) variations between the ring and the volumes inside and outside of the ring, as well as at different angles with respect to a center line of the chamber that is at right angles to the plane of the workpiece holder. These plasma flux variations result in a substantial standard deviation, i.e., in excess of three, of the plasma flux incident on the workpiece. The substantial standard deviation of the plasma flux incident on the workpiece has a tendency to cause non-uniform workpiece processing, i.e, different portions of the workpiece are etched to different extents and/or have different amounts of molecules deposited on them.
Many coils have been designed to improve the uniformity of the plasma. The commonly assigned U.S. Pat. No. 5,759,280, Holland et al., issued Jun. 2, 1998, discloses a coil which, in the commercial embodiment, has a diameter of 12 inches and is operated in conjunction with a vacuum chamber having a 14.0 inch inner wall circular diameter. The coil applies magnetic and electric fields to the chamber interior via a quartz window having a 14.7 inch diameter and 0.8 inch uniform thickness. Circular semiconductor wafer workpieces are positioned on a workpiece holder about 4.7 inches below a bottom face of the window so the center of each workpiece is coincident with a center line of the coil and the chamber center line.
The coil of the ""280 patent produces considerably smaller plasma flux variations across the workpiece than the coil of the ""458 patent. The standard deviation of the plasma flux produced by the coil of the ""280 patent on a 200 mm wafer in such a chamber operating at 5 milliTorr is a considerable improvement over the standard deviation for a coil of the ""458 patent operating under the same conditions. The coil of the ""280 patent causes the magnetic field to be such that the plasma density in the center of the workpiece is greater than in an intermediate part of the workpiece, which in turn exceeds the plasma density in the periphery of the workpiece. The plasma density variations in the different portions of the chamber for the coil of the ""280 patent are much smaller than those of the coil of the ""458 patent for the same operating conditions as produce the lower standard deviation.
Other arrangements directed to improving the uniformity of the plasma density incident on a workpiece have also concentrated on geometric principles, usually concerning coil geometry. See, e.g., U.S. Pat. Nos. 5,304,279; 5,277,751; 5,226,967; 5,368,710; 5,800,619; 5,401,350; 5,558,722 and 5,795,429. However, these coils have generally been designed to provide improved radial plasma flux uniformity and to a large extent have ignored azimuthal plasma flux uniformity. In addition, the fixed geometry of these coils does not permit the plasma flux distribution to be changed for different processing recipes. While we are aware that the commonly assigned copending U.S. application of John Holland for xe2x80x9cPlasma Processor with Coil Responsive to Variable Amplitude RF Envelope,xe2x80x9d Ser. No. 09/343,246, filed Jun. 30, 1999, and Gates U.S. Pat. No. 5,731,565 disclose electronic arrangements for at will controlling plasma flux uniformity for different recipes, the Holland and Gates inventions are concerned primarily with radial, rather than azimuthal, plasma flux uniformity. In the Holland invention, control of the plasma flux uniformity is achieved by controlling a variable amplitude envelope the RF excitation source applies to the coil. In the Gates invention a switch or a capacitor shunts an interior portion of a spiral-like RF plasma excitation coil.
It is accordingly an object of the present invention to provide a new and improved coil for a vacuum plasma processor and method of operating same wherein plasma flux in the processor is relatively uniform.
An additional object of the present invention to provide a new and improved coil for a vacuum plasma processor and method of operating same wherein the plasma density incident on a workpiece of the processor has relatively high azimuthal uniformity.
A further object of the invention is to provide a new and improved coil for a plasma processor, wherein the amplitude variations of standing waves (voltages and/or currents) in the coil are substantially reduced.
In accordance with one aspect of the invention, a coil for a plasma generator of a processor for treating a workpiece includes (1) first and second RF excitation terminals, (2) sufficient length to exhibit transmission line effects for RF excitation of the coil and (3) a capacitor connected to internal locations of the coil on different sides of a discontinuity in the coil. The plasma generator includes a chamber having an inlet for introducing into the chamber a gas which can be converted into the plasma. The coil is adapted to be positioned to couple an RF field to the gas for exciting the gas to the plasma state.
Preferably, the capacitor has an impedance value for the RF excitation such as to cause sudden changes at the location of the discontinuity in amplitude, slope and slope direction of an RF standing wave along the coil.
In the preferred embodiment, (1) the capacitor has an impedance value for the RF excitation, (2) the discontinuity has a location, and (3) the circuitry for supplying the RF excitation to the coil are such that the standing wave voltage along the coil has a voltage polarity change at the location of the discontinuity.
The RF excitation circuitry preferably includes another capacitor and a matching circuit in series with an inductor. The series combination of the matching circuit and the inductor are connected between a first coil excitation terminal and an RF source. The other capacitor is connected between a second coil excitation terminal and a reference potential terminal. The inductor and other capacitor have values for causing approximately equal magnitude and opposite polarity standing wave RF voltages to be at the first and second excitation terminals.
The coil preferably includes plural internal discontinuities and a capacitor is connected to the coil across each discontinuity. Each of the capacitors has an impedance value for the RF excitation such as to cause along the coil, at the location where each discontinuity is located, sudden changes in RF standing wave voltage amplitude, slope and slope direction. Each capacitor has an impedance value for the RF excitation, each discontinuity has a location, and the circuitry for supplying the RF excitation to the coil are such as to cause the standing wave voltage to have a voltage polarity change at the location of each discontinuity.
Preferably the coil includes plural turns. The excitation circuitry and the locations of the discontinuities are such that standing wave voltage polarity reversals occur at locations along the coil displaced from the locations of the discontinuities. The polarity reversals are approximately at the same azimuth angle of the coil in different ones of the turns.
Another aspect of the invention relates to a method of operating a coil that applies an RF plasma excitation field to an ionizable gas. The RF field ionizes the gas to the plasma. The coil has transmission line effects so there is an RF standing wave along the coil between opposite RF excitation terminals of the coil. The method comprises (1) applying an RF excitation voltage to opposite RF excitation terminals of the coil, and (2) suddenly changing by a substantial amount the RF standing wave amplitude and slope and the RF standing wave slope direction at a location along the coil between the excitation terminals.
Preferably, the method also includes suddenly changing the RF standing wave amplitude and slope and the RF standing wave slope direction at plural locations along the coil between the excitation terminals. Each sudden change is such as to reverse the polarity of the RF standing wave.
The RF excitation is preferably applied such that there are approximately equal magnitude and opposite polarity standing wave voltages at the opposite RF excitation terminals.
The method is preferably practiced with a coil having plural turns. The method preferably includes causing the standing wave to have gradual changes in at least some of the plural turns and the sudden changes along at least some of the plural turns. The gradual and sudden polarity reversals along some of the turns preferably are at substantially the same coil azimuthal angle and in the opposite direction at substantially the same azimuthal angle of the coil. A first gradual polarity reversal and a first sudden polarity reversal occur along a first turn, while a second gradual polarity reversal and a second sudden polarity reversal occur along a second turn. The first gradual and second sudden polarity reversals are at substantially the same first azimuth angle of the coil, while the second gradual and first sudden polarity reversals are at substantially the same second azimuth angle of the coil. The polarity reversals occur at azimuthal angles that are equally displaced from each other. One of the turns has a gradual polarity reversal at an azimuth angle different from the sudden polarity reversals.
In accordance with one embodiment of the invention, a variable shunt capacitor is connected to the coil and a variable capacitor is connected in series across a coil discontinuity. The capacitances of the variable capacitors are preferably varied to control the standing wave current and voltage in the two coil segments that are connected together by the series capacitor. To facilitate such control, one electrode of the shunt capacitor is preferably connected to an electrode of the series capacitor. The shunt capacitor creates a standing wave current discontinuity along the coil without introducing a discontinuity in the standing wave voltage along the coil. The series capacitor creates a standing wave voltage discontinuity along the coil without introducing a discontinuity in the standing wave current along the coil.
The previously mentioned Gates patent differs from the present invention because in Gates a capacitor shunts a part of the coil, rather than being connected across a discontinuity of the coil. The Gates patent does not indicate the shunt capacitor causes a sudden slope reversal of a standing wave voltage along the coil. The implication is that the shunt capacitor, which reduces the electromagnetic field in a center portion of the coil, does not reverse the standing wave voltage slope direction. If the shunt capacitor had a large enough value to reverse the standing wave voltage slope direction, no RF electromagnetic field would be derived from the shunted portion of the coil and one of the purposes of FIG. 3 of the Gates patent, i.e., to derive an RF electromagnetic from a center portion of the coil, would be defeated. Also, the Gates shunt capacitor causes increased current variations in different parts of the coil because the current in the coil portion not shunted by the capacitor is responsive to the sum of the currents flowing out of the capacitor and the coil portion the capacitor shunts. The increased coil current variations have a tendency to produce plasma density non-uniformity.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed descriptions of plural specific embodiments thereof, especially when taken in conjunction with the accompanying drawings.